Intestinal phosphate (Pi) absorption in man is abnormally low in many disease states such as chronic uremia, cirrhosis, hypoparathyroidism and diabetes; and as a result of chronic glucocortical therapy, all conditions in which the metabolism of vitamin D may be impaired. In vitamin D-deficient states, the ability to absorb phosphate is increased by treatment with the hormonally most active form of vitamin D3, 1,25-dihydroxy-vitamin D3, (1,25(OH)2D3). When Pi is lowered in the diet, experimental animals increase their active intestinal absorption of Pi, in part by altering 1,25(OH)2D3 metabolism. However, routes by which Pi crosses the intestinal epithelium are not well defined. We plan to study the mechanisms of intestinal Pi absorption by studying altered states of transport such as occur in the pathophysiologic state of vitamin D-deficiency and compare these results to those obtained in 1,25(OH)2D3-repleted and normal animals. To accomplish these goals, the transport of Pi across the intestinal epithelium will be studied under conditions that allow the identification of active transport processes. Also, the rate of phosphate entry into the cells across the brush border will be measured and compared to transport into vesicles prepared from this membrane. Analysis of the kinetics of transport will define the characteristics of cellularly-mediated transport processes. The amount of Pi which diffuses across the mucosal surface of the intestine in response to electrical gradients will allow estimates of the passive permeability to Pi. By comparing the changes in cellular and diffusional transport as a function of vitamin D-treatment, we hope to define the action of this hormone on intestinal Pi transport. Changes in protein composition, enzymatic activities, and Pi transport properties of purified cell membranes will be studied to gain insight into the mechanisms by which the transport alterations occur.